Deposits which contain radioactive elements gradually build up in the cooling systems of nuclear reactors. In order to safely maintain and repair the cooling system, it is necessary to remove these radioactive deposits. This is typically accomplished by treating them with an oxidizing solution, such as one containing an alkaline permanganate. This is followed by treatment with a decontamination solution which is an aqueous solution of a chelate, such as ethylenediaminetetraacetic acid (EDTA),and a solubilizing agent, such as a mixture of oxalic acid and citric acid. The chelate forms a complex with the metal ions from the deposits and solubilizes them, and, thus, prevents them from precipitating out of the solution at another location in the cooling system. The decontamination solution is circulated between the cooling system and a cation exchange resin. The chelated metal ions are deposited on the cation exchange resin, freeing the chelate to solubilize additional metal ions in the deposit.
The difficulty with this decontamination process is that both the chelates and the cation exchange resin compete for the metal ions. As a result, the metal ions do not readily leave the chelate and attach themselves to the ion exchange column. This means that long resin contact times are required, and that the ion exchange column effluent may contain relatively high metal ion concentrations. For example, an effluent concentration of about 200 to about 250 ppm of iron and about 20 to about 30 ppm of cobalt is typical for a decontamination solution consisting of 0.2% EDTA, 0.15% citric acid, and 0.15% oxalic acid. Laboratory data indicates that the presence of these metal ions in the ion exchange resin effluent significantly reduces the level to which the cooling system can be decontaminated. The data shows that while a DF (decontamination factor, which is equal to radioactivity before treatment divided by radioactivity after treatment) of about 20 would be obtainable if these metal ions were removed, a DF of about 14 is actually obtained because iron and cobalt are not effectively removed and the solution radioactivity levels remain high.